A maraging steel is a steel obtained by subjecting a non-carbon or low-carbon steel containing Ni, Co, Mo, Ti and the like in large amounts to solution heat treatment and quenching+aging treatment.
Maraging steels have the following characteristics:
(1) owing to formation of soft martensite in a quenched state, the machinability is good;
(2) owing to precipitation of an intermetallic compound such as Ni3Mo, Fe2Mo and Ni3Ti in the martensite texture during the aging treatment, the strength is very high;
(3) despite high strength, the toughness/ductility is high.
Therefore, maraging steels are used, for example, in an aerospace/aircraft structural material (e.g., engine shaft), an automotive structural material, a high-pressure vessel or a tool material.
Conventionally, a 250 ksi (1,724 MPa) grade 18Ni maraging steel (Fe-18Ni-9Co-5Mo-0.5Ti-0.1Al) has been used for the aircraft engine shaft. However, with the recent desire to improve air pollution, such as tightening of exhaust gas regulations, it is required also for an aircraft to promote the efficiency. In view of engine design, the demand for a high-strength material capable of withstanding high output, downsizing and weight reduction is great.
With respect to such a high-strength material, various proposals have been heretofore made.
For example, Patent Document 1 discloses an ultra-high tensile strength and tough steel containing C: from 0.05 to 0.20 wt %, Si: 2.0 wt % or less, Mn: 3.0 wt % or less, Ni: from 4.1 to 9.5 wt %, Cr: from 2.1 to 8.0 wt %, Mo: from 0.1 to 4.5 wt % or Mo substituted partially or wholly with a double-volume of W, Al: from 0.2 to 2.0 wt %, and Cu: from 0.3 to 3.0 wt %, with the balance being iron and unavoidable impurities.
In this document, it is described that, by adding Cu and Al in combination to a low-carbon Ni—Cu—Mo steel, a strength of 150 kg/mm2 (1471 MPa) or more is obtained without impairing toughness and weldability so much.
Also, Patent Document 2 discloses a high-strength, fatigue resistant steel, containing Ni: from about 10 to about 18 wt %, Co: from about 8 to about 16 wt %, Mo: from about 1 to about 5 wt %, Al: from about 0.5 to about 1.3 wt %, Cr: from about 1 to about 3 wt %, C: about 0.3 wt % or less, Ti: less than about 0.10 wt %, and a balance consisting of Fe and unavoidable impurities, wherein both a fine intermetallic compound and a carbide are precipitated.
In Table 2 of the same patent document, it is demonstrated that such a material has a tensile strength of 284 to 327 ksi (from 1,959 to 2,255 MPa) and an elongation of 7 to 15%.
A maraging steel is generally a high-strength material excellent in the toughness/ductility, but it is known to be difficult to secure toughness/ductility and fatigue resistance in a tensile strength region exceeding 2,000 MPa. Therefore, its application remains at a level that a 250 ksi grade 18Ni maraging steel is used as a general-purpose material.
On the other hand, the steels described in Patent Document 2 is also known as a high-grade general-purpose material. However, in order to meet the requirement for efficiency promotion or the like of an aircraft, it is necessary to more increase the strength (2,300 MPa or more) without causing reduction in the toughness/ductility and fatigue resistance.
[Patent Document 1] JP-A-53-30916 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
[Patent Document 2] U.S. Pat. No. 5,393,488